I am kinda partial to Castrol Select. It is not a complete synthetic. I am still at around 12,000 miles and I do not want to go full synthetic untill around 15,000. I broke it in on 10/40 Valvoline. I also run Castrol Select in my Suburban. It has 86,000 miles and it does like it. I will be going full synthetic in it after I have run a few batches of the half and half.
Synthetic oils can really clean the particles that seal the gaskets, and other sealing points if the engine has run for a long time with conventional Dino.